I would like to argue that having four optional arguments is not very semantic. I would use a command with a key=value
optional argument.
\myquantity[Q_c][i][j][\bar]
is a lot less readable than
\myquantity[accent=\bar,subscript={c,j},lambda subscript=i]{Q}
although the latter might be using a bit too long keys, I hope you agree it is easier to understand.
The following MWE gives an example of your desired command, using a key-value argument. If you indeed think the keys are too long, you can always change them what you find descriptive enough.
\documentclass{article}
\usepackage{amsmath}
\usepackage{pgf}
\makeatletter
\newif\if@myquantity@subscript
\newif\if@myquantity@lambdasubscript
\pgfkeys{
my quantity/.cd,
accent/.code={\let\@myquantity@accent#1},
accent=\relax,
subscript/.code={\def\@myquantity@subscript{#1}\@myquantity@subscripttrue},
lambda subscript/.code={\def\@myquantity@lambdasubscript{#1}\@myquantity@lambdasubscripttrue},
}
\newcommand{\myquantity}[2][]{%
\@myquantity@subscriptfalse
\@myquantity@lambdasubscriptfalse
\pgfkeys{my quantity/.cd,#1}%
\if@myquantity@subscript%
\@myquantity@accent{#2}_{\@myquantity@subscript}%
\else%
\@myquantity@accent{\mathbf{#2}}%
\fi%
(\lambda%
\if@myquantity@lambdasubscript
_{\@myquantity@lambdasubscript}%
\fi)%
}
\makeatother
\begin{document}
\begin{equation*}
\begin{array}{lll}
\myquantity{Q} &
\myquantity[lambda subscript=i]{Q} &
\myquantity[subscript=j]{Q}\\
\myquantity[subscript=j,lambda subscript=i]{Q} &
\myquantity[accent=\bar,subscript=j,lambda subscript=i]{Q} &
\myquantity[accent=\bar]{Q} \\
\myquantity[accent=\tilde]{Q} &
\myquantity[accent=\bar,subscript={c,j},lambda subscript=i]{Q}
\end{array}
\end{equation*}
\end{document}
Resulting in

\lambda
for that to work? If so, why? – Skillmon Aug 3 '18 at 7:28